1. Field of the Invention
The present invention relates to new leuco vat dye preparations in granule form, comprising as essential constituents leuco vat dye, reductant and alkali metal hydroxide.
The invention furthermore relates to the production of the leuco vat dye preparations and their use for dyeing cellulose-containing textile material.
Last but not least, the invention relates to a process for stabilizing dry hydrosulfite against self-ignition.
2. Description of the Background
Indigoid dyes (in particular indigo itself and its bromo derivatives such as brilliant indigo) and anthraquinoid dyes (in partiuclar nitroviolanthrone (C.I. Vat Green 9)) are vat dyes which have been known for a long time and which are employed for dyeing cellulose-containing textile materials.
For dyeing, the water-insoluble vat dye must first be converted by reduction (vatting) into the water-soluble fiber-reactive leuco form, which is then oxidized again to the water-insoluble vat dye pigment after absorption on the material to be dyed.
In the known dyeing processes, the vat dye is vatted in alkaline medium by addition of inorganic reductants such as hydrosulfite (sodium dithionite) and thiourea dioxide or alternatively organic reductants such as hydroxyacetone in a container placed in front of the dye bath. Additional machine-dependent amounts of reductant are used during dyeing, as a part of the leuco dye is oxidized by air contact in the air ducts and on the surface of the dye bath and must be vatted again, which is why reductant must be added to the dye bath.
A disadvantage in the vatting of vat dye using said reductants is the high pollution of the dyeing effluent by sulfate (about 3500 to 5000 mg/l, measured in the effluent of an indigo dyehouse) in the case of hydrosulfite or by oxygen-consuming substances (COD values of about 8000 mg of oxygen/l, measured in the effluent of an indigo dyehouse) in the case of hydroxyacetone.
In the case of indigo, powdered solid to pasty preparations of the leuco form are also described which, as stabilizers, contain polyhydroxy compounds, such as glycerol and in particular compounds derived from sugars, eg. molasses, mixed with alkali or zinc dust (DE Patents 200 914 and 235 047). GB Patent 276 023 furthermore discloses the preparation of powdered leuco dye preparations by heating a mixture of unreduced vat dye, glycol, alkali, hydrosulfite and sodium sulfate and drying in an open vessel and subsequent grinding.
In the case of the first-mentioned preparations, the sulfate content of the dyeing effluent can be lowered effectively, but as a rule these preparations are difficult to meter, as even the dry preparations tend to form lumps, and only dissolve slowly in the dye bath. In the last-mentioned case, the sulfate pollution is increased even further, as sulfate is additionally added to the preparations, which anyway already contain the sulfate produced by the reduction of the dye in the mixture described above.
WO-A-94/23114 discloses a dyeing process in which indigo is employed in prereduced form as an aqueous, alkaline solution obtained during catalytic hydrogenation. In this way, the effluent pollution by organic substances falls, and the sulfate pollution is reduced to the amount of hydrosulfite necessary as a result of air contact during the dyeing process. A disadvantage, however, is that the leucoindigo solution used is sensitive to oxidation and must be handled and stored with exclusion of oxygen. In addition, the high water ballast of this solution is a hindrance both during storage and during transport.
Finally, German Patent Application 43 27 221, which is not a prior publication, describes leucoindigo preparations in granule form, which contain leucoindigo and alkali metal hydroxide as essential components.